I would like to control and power a 6V/90mA pump motor from my Arduino uno without addition power source. As I am pretty new with electronic, after few readings, I came up with a combination of the op-amp + transistor as shown in the following schema:

See the file attached. I can't load the image right now.

I just want to make sure it's the right (and easiest) solution for such purpose.

What PaulS was refering to is the supply you intend to use for the Arduino is going to determine if you can do what you want. The on-board voltage regulator needs of at least 7.0 VDC to 7.5 VDC to output a reliable 5VDC to the Arduino's IC. Therefore you'll need a wall-wart or battery pack with sufficient voltage, probably in the range of 7.5 VDC to 9 VDC.

Whatever the size of the supplied voltage, you can get that voltage from the Vin/RAW pin and drawing the necessary current from that pin won't be a problem (because it's not connected to the IC). However, since this pump is rated for 6VDC you really shouldn't operated it directly from that pin's voltage. Depending on the type and how well constructed it is theoretically the pump will run, but if it does it will run faster than spec'ed and hotter, probably with an appreciably shortened working life as well. If you don't want to add a separate step-down voltage circuit, you'll have to at least include a resitive load to drop the voltage near the pump's rated voltage (which can pull double-duty as a current limiting resistor). I'd only try this if your power supply is reasonable stable, like a regulated AC-to-DC converter, otherwise use a linear regulator chip and associated circuitry similar to what's on the Uno (but for 6VDC not 5VDC).

Well, I was thinking about using a 9V battery to power both arduino + pump.

I am a bit confused now. Where do I get the 6V required from if the Vin can only supply 5V? Besides, I though a op-amp with a negative feedback was enough to regulate the voltage? Should I add also voltage regulator before the pump just to make sure?

If you have a power supply or battery hooked-up to the barrel jack, VIN is the same voltage as the power supply (i.e. VIN = "voltage in"). The only voltage pin that's guaranteed to be 5 VDC is the 5V pin.

Edit: Yes you should have the voltage regulator between the pump and the battery.

To summarize and make sure I understood well, I have 2 options (assuming I use a 9V battery):

I hope your not thinking of one of those little rectangular 9V batteries better suited for smoke detectors. That is a really poor choice for any high current needs (and, yes, that includes a 90 mA pump).

I hope your not thinking of one of those little rectangular 9V batteries better suited for smoke detectors. That is a really poor choice for any high current needs (and, yes, that includes a 90 mA pump).

Unfortunately yes, I was thinking of one of those :-( Either a Lithuim or rechargeable (NiMH) one . Newbie mistake I guess... I did notice that I won't be able to draw enough current from it though, hence, wanted to add a transistor to amplify it.

However I agree with PaulS, if you are going to go with a battery you should use something with a higher maximum charge than a alkaline 9V battery. This charge capacity is measured in ampere-hours (often abreviated Ah), with smaller batteries it's usually expressed as milliapere-hours (mAh). A battery rated at 1 Ah can provide a current of 1 A for one hour. Here's a link to an alkaline battery datasheet from Rayovac (note: the exact specs will vary depending on the manufacturer, but batteries of the same type and chemistry will have similar performance). As you can see although the 9V battery has a higher nominal voltage than one AA, it has approximately a third of the charge (~600 mA vs. ~1800 mA). Since the battery would not only be powering the pump when it's on, but also the arduino continuously, my advice would be to use six AA batteries connected in series (or even six C or D sized as they are all nominally 1.5 VDC).

Battery holders for six AA batteris are readily available online, and might be in stock at a local hobby or hardware store. The majority of them will be designed to have all the batteries connected in series, but you'll probably want to make sure before you buy.